The Mooney Mooney Fault System and Coolac ophiolite suite in the tectonics of the Tumut Trough, southeastern Australia

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Abstract
The Mooney Mooney Fault System and Coolac ophiolite suite separate the Tumut Synclinorial Zone and Young Anticlinorial Zone in southeastern Australia. The Tumut Synclinorial Zone has been interpreted as an inverted rift basin and the Coolac ophiolite suite as obducted oceanic crust. An alternative interpretation is that the Tumut Synclinorial Zone formed by thrust‐tectonic terrane amalgamation. In both cases, the principal deformation was Late Silurian. The Coolac ophiolite suite cannot simply be basin substrate. Mafic magmatism occurred in the trough‐fill stratigraphy when the basin was too narrow to form typical oceanic substrate. The presently associated ‘ophiolitic’ components were partly juxtaposed by tectonism. Because the development of rift basins can stop before typical oceanic crust is formed, the preservation in the geological record of this precursor to oceanic crust is here termed an embryonic ophiolite. The Mooney Mooney Fault System enclosed the Mooney Mooney Terrane of Basden et al. (1985, 1987). This paper discredits the terrane boundaries, establishes the trough‐fill as an overlapping assemblage, and reaffirms the para‐autochthonous tectonic model. Mylonite kinematics support Late Silurian westward upthrusting of the Young Anticlinorial Zone against the Coolac ophiolite suite (possibly during dextral transpression), associated with a deep‐seated tectonic surge affecting the Young Granodiorite. Early Devonian tightening of the Late Silurian upright folding and less ductile reactivation of the fault systems are advocated.